1. Field of the Invention
The present invention relates to an optical transmission technique and, more particularly, to a light-receiving device for converting a signal light pulse string which is multiplexed by optical time-division multiplexing (optical TDM) into an electrical signal, an optical transmission apparatus using the same, and an optoelectronic demultiplexing method.
2. Description of the Related Art
The recent development of technologies related to optoelectronics, including a semiconductor laser, a low-loss optical fiber, an optical fiber amplifier, and a high-speed integrated circuit, allows long-distance transmission of a large amount of information at a data rate of 10 Gb/s. In a coming multimedia age, however, general end users would also utilize a large amount of information such as high-definition video information in real time. Therefore, an infrastructure allowing transmission of a larger amount of information must be established.
In spite of the development of high-speed integrated circuit technologies, electronic devices for processing information at a rate of several tens Gb/s or more have problems of wiring delay, power consumption, and high manufacturing/mounting costs. As a means for transmitting information in a large amount not to allow one-time electronic processing through an optical fiber, optical time-division multiplexing (optical TDM) becomes important as well as optical frequency multiplexing (optical FDM).
Optical TDM transmission is a technique in which a plurality of modulated short light pulse signals are optically multiplexed on the time axis, transmitted, and optically demultiplexed and received on the receiver side. To realize this technique, the short light pulse generation technique, the optical multiplexing technique, the optoelectronic demultiplexing technique, the optical synchronization technique, and the like must be established.
As for the short light pulse generation technique, a stable operation has already been obtained by a means such as gain switching of a semiconductor laser, a mode-locked semiconductor laser, or a soliton light source combining a semiconductor laser with a semiconductor optical modulator. Multiplexing can also be relatively easily realized using a photocoupler for multiplexing light pulse strings while synchronizing each light pulse string with predetermined time slots. To the contrary, a conventional optical demultiplexer has a complex arrangement, which is far from a practical system because of its size, efficiency, cost, and stability.
Various optical demultiplexers for optical TDM have been proposed so far, and they can be roughly classified as follows.
(1) Demultiplexers using nonlinearity (Kerr effect) of an optical fiber.
(2) Demultiplexers using four-wave mixing in a semiconductor laser amplifier.
(3) Demultiplexers which select a specific slot from a passively branched pulse string through a gate.
Typical examples will be briefly described below. As example (1), i.e., a switch using nonlinearity of an optoelectronic fiber, optical demultiplexing using a nonlinear optic loop mirror (NOLM) is known. 1:16 optoelectronic demultiplexing is realized by the NOLM (P. A. Andrekson et al., IEEE Photon. Technol. Lett., Vol. 4, p. 644, 1992).
From the viewpoint of practical use, however, this technique has the following problems. The NOLM has the same arrangement as that of a Sagnac interferometer serving as a high-sensitivity accelerometer. For this reason, the NOLM tends to be influenced by external acoustic vibrations and need countermeasures for the setting environment. In addition, to realize N:N optoelectronic demultiplexing, multiple connection of NOLMs is required. However, since one of the outputs of the photocoupler is identical to an input to the incident port, an optical circulator or the like is necessary, resulting in a complex and bulky arrangement. When a plurality of NOLMs are to be connected, a large number of control pulse sources with a large peak power must be prepared, and their synchronization is also difficult.
As optoelectronic demultiplexing techniques using nonlinearity of an optical fiber, various techniques have been proposed, including an optical Kerr shutter and a method in which cross phase modulation is used to apply frequency shifts in a pulse string and demultiplexing is performed by a diffraction grating. In any case, however, as in the above example, the arrangement becomes complex or tends to be influenced by external disturbance, control light with a large power is necessary, and countermeasures against polarization variation of signal light are necessary. Therefore, it is difficult to realize a compact, low-cost and stable optical demultiplexer.
As example (2) using four wave mixing in a semiconductor laser amplifier, an optical demultiplexer using a polarization insensitive traveling wave type semiconductor laser amplifier in a gain saturation state is known (R. Ludwing and G. Raybon, European Conf. on Optical Comm., 1993, Montreux, Switzerland, ThP 12.2).
This arrangement is simpler than the above optical multiplexer using a NOLM, and the stability is also improved. However, to realize an N:N optical demultiplexer, multiple connection through a narrow-band filter (or optical wavelength demultiplexer) for separating signal light from control light is necessary. In addition, power consumption for control light generation or optoelectronic amplification is large, resulting in a degradation in efficiency. Furthermore, synchronization is difficult. Therefore, in this method as well, it is difficult to realize a compact, low-cost, and stable optoelectronic demultiplexer.
As example (3) using an optical gate switch, an optical demultiplexer using a semiconductor electroabsorption (EA) optical modulator is known (M. Suzuki et al., J. Lightwave Technol., Vol. 10, p. 1912, 1992).
According to this method, the arrangement is simple, and the dependency on polarization is low. However, there are the following problems. First, all light signals are uniformly distributed to N branches through a passive optical coupler. For this reason, the optical power at each branch becomes 1/N. Since light signals except for that corresponding to a predetermined time slot are absorbed in the optical gate, power utilization efficiency is low. Second, to reduce the duty ratio of the optical gate, a high bias voltage and a large sine wave amplitude are necessary, resulting in an increase in power consumption or size of the power supply/driving system of the EA modulator. Third, a large sine wave applied to the EA modulator may influence a low-level signal after reception, so that appropriate electrical isolation must be ensured. Additionally, synchronization between a light signal and an electrical signal must be adjusted branch by branch.
As other techniques included in category (3), there are a method of realizing high-speed optical demultiplexing by a Mach-Zehnder optical modulator (e.g., M. Jinno, IEEE Photon. Technol. Lett., Vol. 4, p. 641, 1992), a method of performing optical demultiplexing by an optical trigger gate (e.g., T. Kamiya et al., CLEO'87 Technical Digest 6, 1987), and the like. Any of them also has similar problems of energy utilization efficiency, gate driving power, and the like. Therefore, in the method (3) as well, it is difficult to realize a compact, low-cost, and efficient optical demultiplexer.
As described above, the conventional optical demultiplexers of an optical TDM system has the following problems, i.e., a complex and bulky arrangement, the requirement of countermeasures against polarization variation of signal light, poor signal light utilization efficiency, large power consumption of a driving signal or control light, and cost and stability, so it cannot be put in practical use.